Sub Pens is a small map, set in a bombed out submarine base, with two sides separated by two Japanese float planes which are positioned in the center. There are several flanking routes, and plenty of cover throughout. Because of this, almost any playstyle can be used effectively. However, it can be difficult to use a sniper rifle, as the map mainly consists of close quarters fighting, and there are few elevated positions which are useful for sniping - effective sniping can only be accomplished if teammates support these few areas with automatic weapons.

In Team Deathmatch, the Marines spawn in the locker room near the docks, while the Imperial Army spawns in the control room on the other side of the map. This room serves as one of the only useful areas for sniping as it holds a slight advantage due to its elevation. Despite this, it is a very small area that would require quick reflexes in order to take down an enemy. This is one of the few maps where shotguns hold an advantage over some weapons.

On spectator mode, players are able to find a dog house with "DAN" written on it.

The mp_docks map was originally to be included on disc. The early map, called Docks, is not related to Sub Pens.

There are Japanese PT boats much like the ones from the mission Black Cats around the map.

Also when panning out in Spectator Mode, one can see caves within a mountain surrounding the Sub Pen. There are also guide lights sticking out of the water (the light disappears if the player hovers directly above it).